(1) Field of the Invention
The present invention relates to a solid-state imaging device used for a digital camera and the like, and especially relates to a solid-state imaging device used for a single-lens reflex digital camera that accepts interchangeable lenses.
(2) Description of the Related Art
In recent years, the market for solid-state imaging devices has expanded remarkably with the spread of digital cameras, camera-equipped mobile phones, and the like. Moreover, single-lens reflex digital cameras that accept various interchangeable lenses from wide-angle to telephoto have become widely available. Meanwhile, there remains a strong demand for slimmer models of digital cameras and the like. A slimmer model means a lens used for a camera part has a short focal length, and light incident on a solid-state imaging device has a wide angle (that is, an angle of incident light measured with respect to an axis perpendicular to an incidence plane of the solid-state imaging device is large).
A solid-state imaging device such as a CCD or MOS image sensor has a two-dimensional array of semiconductor integrated circuits (unit pixels) that each include a light-receiving element, and converts light representing a subject into an electric signal. Since a sensitivity of such a solid-state imaging device is determined by a magnitude of an output current of the light-receiving element in response to an amount of incident light, for a higher sensitivity it is important to ensure that the incident light completely reaches the light-receiving element.
FIG. 1 shows an example of a basic structure of a conventional typical unit pixel 100. As shown in FIG. 1, light (incident light 56 shown by broken lines) vertically incident on a microlens 105 is separated in color by a color filter 2 of any of red (R), green (G), and blue (B), and then converted to an electric signal by a light-receiving element 6. The microlens 105 is used in most solid-state imaging devices, as it contributes to a relatively high light collection efficiency.
However, when the microlens 105 is used, the light collection efficiency decreases depending on an angle of incidence of signal light. In detail, the vertically incident light (incident light 56 shown by broken lines) on the lens can be collected with a high efficiency, but the light collection efficiency decreases for obliquely incident light (incident light 57 shown by solid lines), as shown in FIG. 2. This is because the obliquely incident light 57 is blocked by an Al wiring layer 3 in the pixel and as a result cannot reach the light-receiving element 6.
As mentioned above, a solid-state imaging device is composed of a two-dimensional array of unit pixels. Accordingly, when incident light has a spread angle, an angle of incidence differs between a central unit pixel and a peripheral unit pixel. This causes a problem of a decrease in light collection efficiency of the peripheral unit pixel as compared with the central unit pixel.
FIG. 3 shows an example of a structure of a peripheral unit pixel in a conventional solid-state imaging device 110. Since incident light 58 has a large angle of incidence in the peripheral unit pixel, the Al wiring layer 3 and the light-receiving element 6 are shifted (shrunk) outwardly (toward the periphery) in order to improve the light collection efficiency.
FIG. 2 shows the angle-of-incidence dependence of the light collection efficiency of the conventional solid-state imaging device 110 using the microlens 105. As shown in FIG. 2, though incident light can be collected with a high efficiency when the angle of incidence is about 20° or less, the light collection efficiency drops significantly when the angle of incidence is larger. That is, an amount of light of a peripheral unit pixel is about 40% of that of a central unit pixel in the conventional solid-state imaging device 110, and an overall sensitivity of the solid-state imaging device 110 is limited by a sensitivity of the peripheral unit pixel. Besides, the overall sensitivity of the solid-state imaging device 110 further decreases with a decrease in pixel size, making it extremely difficult to apply to an optical system with a short focal length such as a small-size digital camera. Furthermore, it is impossible to perform a more circuit shrinkage than the present level in a manufacturing process.
To solve the above problems relating to wide-angle incident light, a solid-state imaging device that realizes a gradient index lens with an effective refractive index by forming a fine structure equal to or smaller than a wavelength of incident light has been proposed (for example, see pamphlet of International Patent Publication WO 05/101067, hereafter referred to as Patent Reference 1). In more detail, in a central part of an imaging area in the solid-state imaging device, a gradient index lens having an effective refractive index distribution that is symmetrical about a center of a corresponding unit pixel is formed by a combination of a plurality of concentric zone areas obtained as a result of dividing by a line width equal to or smaller than the incident light wavelength. In a peripheral part of the imaging area in the solid-state imaging device, on the other hand, a gradient index lens having an effective refractive index distribution that is asymmetrical about a center of a corresponding unit pixel is formed by a combination of a plurality of concentric zone areas obtained as a result of dividing by a line width equal to or smaller than the incident light wavelength, with a center of the concentric zone areas being displaced (offset) from the center of the unit pixel. According to this technique, even when light is obliquely incident on the imaging area peripheral part in the solid-state imaging device at a large angle with respect to an axis perpendicular to an incidence plane, the incident light can be collected at a light-receiving element, with it being possible to achieve a same level of sensitivity as in the imaging area central part in the solid-state imaging device.
A solid-state imaging device 210 which employs the technique disclosed in Patent Reference 1 includes a gradient index lens having a different effective refractive index in each of an imaging area central part, an imaging area intermediate part (located between the center and the periphery), and an imaging area peripheral part, as shown in FIGS. 5A, 5B, and 5C. This being so, in the case where an imaging lens 220 for wide-angle incident light is used in a single-lens reflex digital camera 200 as shown in FIG. 4, even when light is obliquely incident on the imaging area peripheral part at a large angle with respect to the axis perpendicular to the incidence plane, the incident light can be collected at the light-receiving element 6. Hence a same level of sensitivity as in the imaging area central part can be achieved in the imaging area peripheral part.
However, the single-lens reflex digital camera 200 not only uses a lens for wide-angle incident light, but also uses an imaging lens 230 for light that is telecentrically (that is, chief rays are approximately parallel to an optical axis) incident on the solid-state imaging device 210 as shown in FIG. 6.
When light is telecentrically incident on the solid-state imaging device 210 which includes the gradient index lenses suitable for wide-angle incident light as shown in FIGS. 5A, 5B, and 5C, the light is bent more than necessary and as a result the amount of light reaching the light-receiving element 6 decreases in the imaging area peripheral part, as shown in FIGS. 7A, 7B, and 7C. This causes a peripheral part of an image to become dark.
In view of the above problems, the present invention has an object of providing a solid-state imaging device and the like that can capture an image which is bright through to its periphery, even when used in, for example, a single-lens reflex digital camera that accepts various interchangeable lenses from wide-angle to telephoto.